1. Technical Field
The present application generally relates to methods of forming carbon nanotube-based contacts to semiconductors and more particularly to method of manufacturing carbon-nanotube based contacts to semiconductors for electronic and photonic applications.
2. Discussion of Related Art
III-V semiconductors, specifically GaN (Eg˜3.37 eV), are widely employed for optoelectronic and electronic devices. These materials have many advantages compared to silicon based technology. However, the lack of a stable oxide high manufacturing costs of the III-V semiconductors have prevented them from replacing silicon electronics beyond niche devices and optoelectronic devices (a substantial market itself).
Carbon nanotubes (CNTs) are a new material that is finding various uses especially in the nanoelectronics industry. CNTs contain a tubular structure with diameters ranging from 1-2 nm and have many unique and valuable properties such as dual metallic and semiconductor conduction, high mechanical strength (modulus around 1 TPa), excellent thermal and optical properties and chemical inertness/resilience.
A major challenge in the optoelectronics industry is the ability to form an ohmic, low contact resistance between a metallic (or conductive metal-oxides) thin film and the active p-GaN substrate, which has a very low conductivity. The metal contact must also have a high optical transmittance at the desired operation wavelength, which mandates the use of ultra-thin metal films (<50 nm). These thinner metal films may lead to current uniformity and current injection problems into the p-GaN. Therefore, a balance must be struck between the thin film sheet resistance, the thickness of the metal film and the metal film's optical transmittance. Ideally, a metal or conductive metal-oxide film should form an ohmic contact to the p-GaN, have a contact resistivity of <10−3 Ohms-cm2, have a sheet resistance of <100 Ohms per square, have an index of refraction >1.5 and have an optical transmission of >85% at the desired wavelength. Further complicating the development of GaN optical devices are the many issues with processing p-GaN; such as, sensitivity of the active region to photoresists, dry etching and some solvents.
Currently, a scalable, manufacturable process does not exist for making CNT-based contacts to p-GaN devices. For a process to be manufacturable, it should be useable with the tools and procedures employed in industry. The process outlined in this invention not only conforms to such guidelines, but results in encapsulation of the GaN surface during processing thereby maximizing the quality of the interface of the product.
The idea of using nanotubes to make contact to p-GaN has been published by Lee, K., at al. (“Single wall carbon nanotubes for p-type ohmic contacts to GaN light-emitting diodes”, Nano Letters, 4(5), (2004), 911-914) and patented by Rinzler, A. G., et al, (“Semiconductor device and method using nanotube contacts” US Application number: US 20050199894 (2205)), which both use a thick layer of carbon nanotubes to make a low contact resistance contact to a GaN substrate that was able to transmit blue light centered around 434 nm. However, neither of these references present or purport to have implemented a process suitable in a manufacturing environment. More particularly, in both references, the type of nanotubes and the technique employed to deposit the fabric cannot be reproduced in a suitable manufacturing process. Both the publication by Lee, et. al, and the patent by Rinzler et. al report the use of metal lift-off, which will degrade the device characteristics since the active region is exposed to photoresist, basic developers and solvent chemistries, which damages the active region of the p-GaN, a very sensitive material. The present disclosure details techniques that avoids the abovementioned difficulties and provides new methods of making a desired contact besides using just a nanotube fabric.